Repression of FasL expression by retinoic acid involves a novel mechanism of inhibition of transactivation function of the nuclear factors of activated T-cells.

Retinoids are potent immune modulators that inhibit Fas ligand (FasL) expression and thereby repress the activation-induced apoptosis of immature thymocytes and T-cell hybridomas. In this study, we demonstrate that all-trans -retinoic acid (all-trans -RA) directly represses the transcriptional activity of the nuclear factors of activated T-cells (NFAT), which is an important transactivator of the FasL promoter. The analysis of reporter constructs containing the FasL promoter and wild-type or mutant NFAT binding-sites indicated that all-trans -RA repression was mediated via an NFAT binding element located in the promoter. A reporter construct comprising the NFAT binding sequence linked to a heterologous SV-40 promoter showed that NFAT transcriptional activity was significantly inhibited by all-trans -RA. Furthermore, all-trans -RA inhibited activation of the distal NFAT binding motif present in the interleukin (IL)-2 promoter, suggesting that the inhibition of NFAT function by all-trans -RA was not specific to the FasL promoter. Gel shift assays corroborated the results of the gene reporter studies by showing that all-trans -RA decreased the NFAT binding to DNA. All-trans -RA blocked translocation of NFATp from the cytosol into the nucleus, which was induced by PMA/ionomycin treatment in HeLa cells transfected with a Flag-tagged NFATp. Taken together, our results indicate that FasL inhibition by all-trans -RA involves a novel mechanism whereby the transcriptional function of NFAT is blocked. [ABSTRACT FROM AUTHOR]

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